The present invention relates to a voltage detector for detecting the voltage developing in a selected area of an object to be measured such as an electric circuit. In particular, the present invention relates to a voltage detector of the type that detects voltage by making use of the change in light polarization that occurs in accordance with the voltage developing in a selected area of an object to be measured.
Various voltage detectors have been used to detect the voltage developing in a selected area of objects to be measured such as electric circuits. Conventional voltage detectors are roughly divided into two types: in one type, the probe is brought into contact with a selected area of an object to be measured and the voltage developing in that area is detected; and in the other type, the probe does not make contact with a selected area of an object to be measured and instead an electron beam is launched into that area and the voltage developing in it is detected.
Voltage changes rapidly in fine-line portions of objects such as integrated circuits that are small and complicated in structure, and a strong need exists in the art for detecting such rapidly changing voltage with high prevision without affecting the fine-line portions. However, this need has not been fully met by the prior art voltage detectors. With devices of the type that detects voltage by bringing the probe into contact with a selected area of an object to be measured, it is difficult to attain direct contact between the probe and a fine-line portion of the object of interest such as an integrated circuit. Even if this is successfully done, it has been difficult to correctly analyze the operation of the integrated circuit solely on the basis of the voltage information picked up by the probe. A further problem involved is that contact by the probe can cause a change in the operation of the integrated circuit. Voltage detectors of the type that employs an electron beam has the advantage that they are capable of voltage detection without bringing the probe into contact with an object to be measured. However, the area to be measured with such voltage detectors has to be placed in vacuum and its surface must be exposed at that. In addition, the area to be measured is prone to be damaged by electron beams.
The prior art voltage detectors have a common problem in that they are unable to operate quickly enough to follow rapid changes in voltage and hence fail to achieve precise detection of voltages that change rapidly as in integrated circuits.
With a view to solving these problems, it has been proposed in Japanese Patent Application No. 280498/1986 that voltage be detected by making use of the polarization of a light beam that changes with the voltage developing in a selected area of an object to be measured.
A voltage detector using this operating principle is schematically shown in FIG. 5. The detector generally indicated by 50 is composed of the following elements: an optical probe 52 connected to an optical fiber 51; a light source 53 in the form of a laser diode; a polarizer 54 that receives the light beam issuing from the light source 53 and which transmits only the portion that has a predetermined polarized component; a beam splitter 56 that splits the light beam from the polarizer 54 into two components, one of which is to be launched into the optical probe 52 while the other component is to be launched into a photoelectric converter 55; a beam splitter 59 that directs the input light from the beam splitter 56 toward the optical probe 52 while launching the input light from the probe 52 into an analyzer 57 and a photoelectric converter 58; a collimator 60 disposed between the beam splitter 59 and the optical fiber 51; and a comparator circuit 61 for comparing the electric signals produced from the photoelectric converters 55 and 58.
The optical probe 52 is filled with an electro-optic material 62 such as an optically uniaxial crystal of lithium tantalate (LiTaO.sub.3). The tip 63 of the electro-optic material 62 is worked into a frustoconical shape. The optical probe 52 is surrounded with a conductive electrode 64 and has a coating of thin metal film 65 on its tip 63.
Voltage detection with the system shown in FIG. 5 starts with connection the conductive electrode 64 on the circumference of the optical probe 52 to a predetermined potential, say, the ground potential. Then, the tip 63 of the probe 52 is brought close to an object to be measured such as an integrated circuit (not shown), whereupon the thin metal film 65 coated on the tip 63 acquires a certain potential on account of a given voltage developing in the object to be measured. The difference between the potential of the thin metal film 65 and the ground potential to which the conductive electrode 64 is connected produces a change in the refractive index of the tip 63 of the electro-optic material 62. Stated more specifically, the difference between refractive indices for ordinary ray and extraordinary ray in a plane perpendicular to the optic axis will change in the optically uniaxial crystal.
The light beam issuing from the light source 53 is polarized by the polarizer 54 and a predetermined polarized component of intensity I is launched into the electro-optic material 62 in the optical probe 52 via beam splitters 56, 59, collimator 60 and optical fiber 51. Reference light produced by splitting with the beam splitter 56 has an intensity of I/2, and the input light that is launched into the electro-optic material 62 after being split with the beam splitters 56 and 59 has an intensity of I/4. As already mentioned, the refractive index of the tip 63 of the electro-optic material 62 varies with the potential of the thin metal film 65, so the input light launched into the electro-optic material 62 will experience a change in the state of its polarization at the tip 63 in accordance with the change in the refractive index of the latter. The light is then reflected from the thin metal film 65 and makes a return trip through the electro-optic material 62, from which it emerges and travels back through the optical fiber 51. If the length of the tip 63 of the electro-optic material 62 is written as l, the state of polarization of input light launched into that material will change in proportion to the difference between refractive indices for ordinary ray and extraordinary ray and to the length 2l as well. The output light sent back into the optical fiber 51 travels through the collimator 60 and beam splitter 59 and enters the analyzer 57. The intensity of the output light entering the analyzer 57 has been decreased to I/8 as a result of splitting with the beam splitter 59. If the analyzer 57 is designed in such a way as to transmit only an output light beam having a polarized component perpendicular to that extracted by the polarizer 54, the intensity of output light that is fed into the analyzer 57 after experiencing a change in the state of its polarization is changed from I/8 to (I/8)sin.sup.2 [(.pi./2)V/V.sub.0 ] in the analyzer 57 before it is further fed into the photoelectric converter 58. In the formula expressing the intensity of output light emerging from the analyzer 57, V is the voltage developing in a selected area of an object to be measured, and V.sub.0 is a half-wave voltage.
In the comparator circuit 61, the intensity of reference light produced from the photoelectric converter 55, or I/2, is compared with the intensity of output light produced from the other photoelectric converter 58, or (I/8)sin.sup.2 [(.pi./2)V/V.sub.0 ].
The intensity of output light, or (I/8)sin.sup.2 [(.pi./2)V/V.sub.0 ], will vary with the change in the refractive index of the tip 63 of the electro-optic material 62 that occurs as a result of the change in the potential of the thin metal film 65 coated on the tip 63 of the optical probe 52. Therefore, this intensity can be used as a basis for detecting the potential of the thin metal film 65, or the voltage developing in a selected area of an object to be measured, say, an integrated circuit.
As described above, in using the voltage detector 50 shown in FIG. 5, the tip 63 of the optical probe 52 is brought close to an object to be measured and the resulting change in the refractive index of the tip 63 of the electro-optic material 62 is used as a basis for detecting the voltage developing in a selected area of the object of interest. Therefore, the voltage developing in fine-line portions of a small and complicated object such as an integrated circuit which are difficult to be contacted by a probe or which cannot be contacted by the same without affecting the voltage to be measured can be effectively detected by the detector 50 without bringing the optical probe 52 into contact with such fine-line portions.
The voltage detector 50, however, has its own problems. First, the state of polarization of a light beam issuing from the polarizer 54 changes as it passes through the optical fiber and thereby the input light launched into the electro-optic material 62 contains a polarized component in addition to the linearly polarized component extracted by the polarizer 54. Secondly, the state of polarization of output light that emerges from the electro-optic material 62 after being reflected from the thin metal film 65 is also distorted as it passes through the optical fiber 51 and the output light entering the analyzer 57 contains a polarized component that is unwanted for voltage detection purposes.
For the reasons stated above, it is difficult to extract in the analyzer 57 and the photoelectric converter 58 the polarized component that is dependent solely on the change in the state of polarization that has occurred in the electro-optic material 62 in the optical probe 52, and this difficulty has reduced the detection precision attainable by the system shown in FIG. 5.